1. Technical Field
The present invention relates to seismic sensing; and more particularly, to seismic surveying of an earth formation in relation to a borehole.
2. Description of Related Art
Seismic surveying is a standard tool for the exploration of hydrocarbon reservoirs. Traditional seismic surveys have been consistently performed using geophones and hydrophones. Geophones measure the earth media""s particle displacement or particle velocity, while hydrophones measure the fluid pressure changes due to a remote source in boreholes or in a marine environment. The seismic geophones and/or hydrophones are typically temporarily deployed along the earth""s surface or along the ocean bottom to perform surface seismic surveys, or in a borehole to perform vertical seismic profiles, or cross-well seismic measurements.
An alternative seismic surveying tool is disclosed in commonly assigned U.S. patent application Ser. No. 08/800,208, Fiber Optic Bragg Grating Sensor System for Use in Vertical Seismic Profiling, filed Feb. 12, 1997, the contents of which is incorporated herein in its entirety. The invention disclosed in the ""208 application comprises a fiber optic sensor positioned within a fluid filled metal capillary tube. The capillary tube is lowered inside of a borehole wherein the sensor is responsive to strain on the capillary tube, through the incompressible fluid, caused by acoustic pressure associated with a seismic pressure wave. Like the prior art described herein above the ""208 fiber optic sensor is described as being temporarily deployed in a borehole to perform vertical seismic profiles. Further, the ""208 sensor measures the strain response of the capillary tube to a seismic event in relation to the borehole, and therefore the earth formation, and does not measure the response of the earth formation directly.
These prior art seismic surveys are optimized for short term exploration and production objectives but can be quite costly, especially when well production needs to be shut down for the measurements, or a large area needs to be explored, or the surveys need to be repeated to monitor reservoir changes over time. There is a need for a new approach to acquire direct earth formation seismic wave data because of the extensive and repeated use of surface, ocean bottom, and in-well seismic techniques to image and monitor earth formations and reservoirs.
The present invention provides a new and unique method and apparatus for performing a seismic survey of an earth formation.
The seismic survey method includes arranging at least one combined strain seismic sensor and borehole structure having a strain sensor arranged therein into the borehole; providing a seismic disturbance in relation to the borehole; receiving a combined strain seismic sensor and borehole structure signal containing information about the seismic disturbance in relation to the borehole; and providing seismic survey information about the earth formation in relation to the borehole depending on the information contained in the optical seismic sensor and borehole structure signal.
The combined strain seismic sensor and borehole structure may include one or more optical seismic sensors in combination with one or more borehole structures, including either a flexible carrier film having the optical fiber arranged therein or thereon, or either coiled tubing, a production tube or a well casing having the optical fiber wrapped therein or thereabout, or a combination thereof wherein the combination is closely coupled to the earth formation. The flexible carrier film may be deposed in or on a packer/bladder, or other type of coupling mechanism, arranged between the production tube and the well casing also in the borehole.
The optical fiber may have a Fiber Bragg Grating sensor therein for sensing the seismic disturbance. The Bragg grating sensor may include either a Bragg grating point sensor, multiple Bragg gratings, or a lasing element formed with pairs of multiple Bragg gratings. Based on the principle of Fiber Bragg Grating sensors, these sensors can be made to measure the deformation of the earth formation over the length of a Fiber Bragg Grating sensor. To put it precisely, a Fiber Bragg Grating sensor directly measures the strain of the earth material at the sensor location. This provides new ways to perform seismic surveying using strain measurements.
Strain seismic data, as acquired by a Fiber Bragg Grating sensor array, can yield the same information as traditional geophone data when used for seismic image processing. Based on similar principles, optical fiber without Fiber Bragg Gratings, can also be used to measure the deformation of the earth formation over the optical fiber depending on the change of length of the optical fiber. In effect, the techniques for sensing the changes in the length of the optical fiber as a function of the seismic disturbance may be done with or without the use of a Fiber Bragg Grating in the optical fiber. When using a Bragg Grating sensor, the change of length of the optical fiber may cause a strain induced shift (xcex94xcex) in the Bragg Grating sensor that causes a change in an optical parameter which is sensed by a light source, detection measurement and signal processor device. In the case of an interferometer based sensor, the change in length (xcex941) of the optical fiber produces a time of flight change which is sensed by a light source, detection measurement and signal processor device.
The optical fiber may be arranged in a hoop strain fiber loop (horizontal), an axial strain fiber loop (vertical), an oblique loop (angled), or any combination thereof, on or in the flexible carrier film, the coiled tubing, the production tube, the well casing, or a combination thereof, for sensing the seismic disturbance in relation to the axis of the borehole. In effect, the seismic disturbance results in strain in the earth formation that is coupled to the flexible carrier film, the coiled tubing, the production tube, the well casing, or the combination thereof at some appropriate angle.
The optical fiber sensors may be configured using any type of optical grating-based measurement technique, e.g., scanning interferometric, scanning Fabry Perot, acousto-optic tuned filter, time of flight, etc. having sufficient sensitivity to measure the strain response of the borehole structure in terms of changes in the length of the optical fiber as a function of the seismic disturbance.
The seismic survey apparatus features the light source, detection measurement and signal processor device in combination with the combined optical seismic sensor and borehole structure, which cooperate as follows:
The light source, detection measurement and signal processor device provides the optical signal to the combined optical seismic sensor and borehole structure. The light source, detection measurement and signal processor device responds to the combined optical seismic sensor and borehole structure signal from the combined optical seismic sensor and borehole structure, for providing seismic survey information about the earth formation in relation to the borehole depending on the information contained in the combined optical seismic sensor and borehole structure signal.
The combined optical seismic and borehole structure sensor responds to the optical signal from the light source, detection measurement and signal processor device, and also responds to a seismic disturbance in relation to the earth formation, for providing the combined optical seismic sensor and borehole structure signal to the light source, detection measurement and signal processor device. The combined optical seismic sensor and borehole structure signal contains information about the seismic disturbance in relation to the borehole and earth formation.
One advantage of the present invention is that seismic sensors can be permanently implanted in the borehole to allow seismic imaging/monitoring over time. Another advantage is that the seismic sensors are coupled to the earth formation and measure the direct strain response of the earth formation to a seismic event. Yet another advantage is that a large number of seismic sensors can be deployed to allow high resolution measurement and effective data processing, and also can be designed to be low profile, which minimizes the additional space occupied by the seismic sensor.
The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawings.